The embodiments disclosed herein relate to dynamic mechanical analyzers (DMAs) and to sample fixtures for use in DMAs. Dynamic mechanical analysis is a material characterization method which exposes a sample to a periodic force and measures the resulting displacement. A dynamic mechanical analyzer measures the phase between the applied force and the resulting displacement of the sample, and uses this measurement to separate the force into a viscous and an elastic component. The elastic component is the displacement energy that is stored in the sample while the viscous component is the displacement energy that is lost via viscous dissipation.
In many DMAs, the instrument typically displays the elastic and viscous components as the storage modulus and the loss modulus. Experiments are generally performed over a range of temperatures or over a range of frequencies to measure temperature-dependent or frequency-dependent material characteristics, respectively. The glass transition is an important temperature and frequency dependent material characteristic that is often measured using dynamic mechanical analysis. In this transition, an amorphous or glassy material becomes more flexible or rubbery, typically as temperature increases. DMAs typically detect the glass transition as a sharp decrease in the storage modulus and an attendant increase in the loss modulus.
DMAs may subject samples to different deformation modes, including tension, compression, shear and flexure. The tension mode is typically employed for analysis of samples with very small cross sectional areas such as thin films or fibers. The compression mode is most often used for analysis of very soft materials such as foams. FIGS. 8 and 11 of U.S. Pat. No. 5,710,426, which is incorporated by reference herein, show examples of tension and compression fixtures. Samples may be deformed in shear using torsion fixtures or parallel plate shearing fixtures. FIG. 12 of U.S. Pat. No. 5,710,426 shows an example of a parallel plate shearing mode fixture. Flexural deformation mode is commonly used for samples such as composites that are very stiff. Flexure modes include 3-point bending, 4-point bending and single and double cantilever modes. FIGS. 9 and 10 of U.S. Pat. No. 5,710,426 show examples of 3-point bending and double cantilever fixtures. In single and double cantilever modes, the ends of the sample are fixed to prevent rotation, causing the sample to develop an s curve when displaced.
In the cantilever flexure modes, the samples are generally parallelepipeds having a rectangular cross section that is usually wider than it is thick, i.e. the thin dimension is oriented parallel to the displacement direction. To obtain accurate measurements of the moduli, the ends of the sample must be rigidly clamped to prevent any rotation; even very small rotations may result in substantial errors in measured moduli. The fixtures of FIG. 9 of U.S. Pat. No. 5,710,426 achieve this for double cantilever mode by using a rigid closed frame that joins the clamping surfaces of the two fixed ends together. While this design is very effective at preventing rotation of the ends of the sample, it has the disadvantage of constraining the sample, preventing its expansion and contraction as it is heated or cooled. This causes thermal stresses and strains to develop in the sample that cause errors in the measured moduli.
One fixture that reduces thermal expansion and contraction strains uses a single cantilever mode in which the sample is clamped at one end and the opposite end is clamped by the moving fixture. This has the disadvantage that the moving fixture is attached to the DMA drive rod and depends on the stiffness of the drive rod and fixture attachment to prevent rotation. However, it often does not adequately prevent rotation of the moving end of the sample, especially when stiff samples are measured. This reduces the accuracy of the measured stiffness and hence the moduli. This method is only suitable for samples having low stiffness, which are less likely to experience rotation of the moving end of the sample.
Another approach for reducing thermal expansion strains is disclosed in U.S. Pat. No. 4,967,601, which is incorporated by reference herein. This patent discloses a resilient sample holder that can move in a direction perpendicular to the displacement of the sample to accommodate the sample thermal expansion and contraction. The fixed ends of the sample fixtures are mounted to flexible arms whose bending characteristics allow the sample fixtures to move. The flexibility of these arms also reduces the resistance to rotation of the fixed ends of the sample fixtures, introducing similar errors to the single cantilever fixtures.
Yet another means for reducing thermal expansion strains is disclosed in U.S. Pat. No. 4,730,498, which is incorporated by reference herein. This patent discloses a double cantilever fixture having the fixed ends mounted on carriages that allow them to move readily in a direction perpendicular to the displacement of the sample to accommodate sample thermal expansion and contraction. However, this fixture is quite massive and requires bearings that must operate with very low friction even at elevated temperatures. The massive fixture may create large temperature lags, which could adversely affect sample measurement accuracy.